On an inverter side of a DC transmission system, a current converter performs commutation processing according to a maximum trigger delay angle. The commutation processing refers to a physical process of switching a current flowing through the current converter from one current path to another current path by opening or closing a valve group when the current converter is in operation.
In the related art, a DC current instruction value is adopted to determine a commutation voltage drop and a positive slope coefficient K is introduced to ensure a positive slope relationship between the DC voltage and the DC current of the current converter. When small transient disturbance occurs in the DC transmission system, the DC current instruction value stays the same, and the actual DC current is increased due to the disturbance. In this case, if a maximum trigger delay angle that is determined in the related art and is larger than the maximum trigger delay angle before the disturbance is adopted, the actual DC voltage value on the inverter side of the DC transmission system is increased, so that the actual DC current on the inverter side of the DC transmission system is reduced, and thus the DC transmission system recovers to a steady operating point.
However, when large transient disturbance occurs in the DC transmission system, the solution of determining a maximum trigger delay angle in the related art may cause a commutation failure of the current converter or even blocking of the current converter.